[xsl] select lang attribute problem

any ideas on selecting nodes based on the 2 following conditions being met

a) existance of lang attribute is boolean false ( in other words when
there is no explicit lang attribute select it as a default)

if a) is TRUE then

b) presence of lang="en" is boolean true ( the 'en' would be supplied by
a xsl:param)

a solution at the top level of xsl would be most desirable with an
xsl:param setting the desired language to extract ............ so far
i've got lots of methods,
just can't find anything elegant enough to apply to a larger system for
multilingual support. any comments or pointers with how people are
approaching multilingual xml/xsl..greatly appreciated.